It is known to use infra-red densitometry to measure the variations in the optical density of a moving web. British Patent Specification GB-A-1364439 discloses such a method which comprises illuminating a spot on the moving web with a source of infra-red radiation and using a photosensitive detector positioned on the opposite side of the web from the source to measure the diffused radiation issuing from the web. A radiation-absorbing screen is used to prevent specular radiation from reaching the detector. The energy impinging on the detector is related to the distance of the web from the detector. The optical density of the web can then be determined from the level of radiation received by the detector. The arrangement is such that the optical density measurement is not affected by any vibrations produced in the moving web.